import java.io.*;
public class triangle_area{
	public static void main(String args[]) throws IOException{
		InputStreamReader in=new InputStreamReader(System.in);
		BufferedReader bin=new BufferedReader(in);
		double a,b,c,s,area;
		a=Double.parseDouble(bin.readLine());
		b=Double.parseDouble(bin.readLine());
		c=Double.parseDouble(bin.readLine());
		s=(a+b+c)/2.0;
		area=Math.sqrt(s*(s-a)*(s-b)*(s-c));
		System.out.println("area="+area);

	}
}